If you create Item-Matrix products in Lightspeed, these will be sent to Amazon as variants with drop-down selections. Similar to how you map categories in Lightspeed to categories in Amazon, you need to also map the attribute set and attribute value to its corresponding schema in Amazon.
To do this, access your Hyperspace settings and click on Amazon settings.
Once this screen opens, click on the Match Amazon Categories to Lightspeed tab.
In this section you will see a place to map your Lightspeed categories to Amazon, but you will also see a place to select the schema. If you have products in this category that are Item-Matrix products in Lightspeed, you should select a schema for those products.
First locate your category and click Set.
First you select the Amazon Category Schema that best represents your product category. Once that has been selected, choose the Product Type that best represents your product category.
Next choose the Amazon Variation theme (Amazon Attribute Set) that best fits products in this category.
Finally choose your matching Lightspeed settings.
Choose the Lightspeed Attribute set that best matches the Amazon Variation Theme.
Choose the attribute values in Lightspeed that are associated with this Attribute set.
You can now re-sync your product to Amazon.